Everyone who has come across Keane, worked with him, or played with and against him, can vouch for the fact Manchester United's iconic captain has never taken any prisoners.

Whether it's manager and mentor Sir Alex Ferguson, other Old Trafford colleagues and Manchester United's own fans, or various members of football's worldwide community - all have at one time or another fallen foul of this most driven of competitors.

And that's been primarily because of his relentless pursuit of perfection.

Keane has made enemies in establishing himself as the man Ferguson rates as the greatest player of his Old Trafford reign.

But no-one with United's best interests at heart will be happy to celebrate the passing of a legendary Old Trafford figure.

The breaking of the bond between Ferguson and his captain - and it does appear to have ultimately broken down, despite Keane for so long Ferguson's voice on and off the pitch - may now also signal the final changing of the guard at Old Trafford.

How long will it be before Ferguson himself follows his favourite footballing son out of the door?

Ferguson has rarely had cause to regret the day he took Keane from under the nose of a fuming Kenny Dalglish for £3.75m in July 1993, just as he was poised to join Blackburn from Nottingham Forest.

Keane came to embody Ferguson's own spirit inside Old Trafford - issuing his orders on the pitch and delivering public criticism that many suspected had been scripted by his manager.

He has been central to Ferguson's whole football philosophy and arguably the Premiership's finest midfield operator in the last decade.

It is no co-incidence that it has not just been the arrival of Roman Abramovich's chequebook that has resulted in United's failure to maintain their Premiership supremacy.

Keane's own declining powers and physical fitness have also been a major factor in Ferguson's struggle to return the title to Old Trafford.

And Ferguson's failure to adequately address the issue of Keane's potential replacement is a huge flaw in his future planning.

Keane has been embodied Ferguson's spirit

As for Keane himself, spats and silverware have come in equal measure, his row with Republic of Ireland boss Mick McCarthy and a lack of repentance over a horror tackle on Alf Inge Haaland evidence of his darker side.

He has publicly lashed his own team-mates with strategically-timed attacks, and his now-infamous attack on Old Trafford's "prawn sandwich" supporters created a new term in football's glossary.

And yet it was this verbal assault that cemented Keane's status as United's man of the people, a player more at home with the fan on the street than in football's more corporate corridors.

For all that controversy, Keane deserves to leave Old Trafford with his status as one of the game's greats unchallenged.

He emerged at Nottingham Forest under Brian Clough as a wild, driving, goal-scoring midfield man with huge untapped potential.

ROY KEANE QUOTES

I have said I wouldn't mind finishing my career at Celtic but as long as United want me, I am happy

On his future in 2000

They have a few drinks and probably the prawn sandwiches and don't realise what's going on out on the pitch. I don't think some people who come to Old Trafford can spell football, never mind understand it

On some of Man Utd's fans

The great teams get back to finals and win it, and this just shows we are not a great team. We're just an average team in a lot of areas

On his team-mates in 2001

Even in the dressing
room afterwards I had no remorse. What goes around comes around. He got his just rewards. My attitude is an eye for an eye

On his tackle on Alf-Inge Haaland

I've been fortunate to work under two of the game's great managers, in Brian Clough and Alex Ferguson, and the thought of being manager of Man Utd would be fantastic

On management in 2003

Our performance levels have not been good enough. Everyone at this club needs to look at themselves and ask whether they are giving 100%

After failing to win a trophy in 2005

He started at Old Trafford in the shadow of Bryan Robson, but soon emerged as the finished article, helped by the shrewd guidance of Ferguson.

And, showing an astute football brain, he has tailored his style to adjust to the passage of time and the onset of injuries.

Recently he has been more a restrained holding player and enforcer rather than a rampaging force, but still outstanding in his field.

His finest hour may also remain as his biggest disappointment.

No United fan will ever forget Keane's stunning performance against Juventus in the Champions League semi-final victory in Turin in 1999 - delivered after picking up a yellow card that ruled him out of the final against Bayern Munich.

This is what makes Ferguson's task of finding a replacement so difficult.

Of course, the perfect model is Liverpool's Steven Gerrard, but Ferguson has long accepted that this is one prize that will always remain beyond his grasp.

Patrick Vieira may have been a contender in the past, but he has settled for Serie A and Juventus.

Former Everton midfield man Thomas Gravesen has been linked with a move to Old Trafford, but he would never exert Keane's influence and is now edging towards 30.

The prime contender must be Bayern Munich's Michael Ballack - gifted and with the arrogance and ability to believe he can not only equal, but surpass, Keane's influence.

The fear for Ferguson will be that in Keane, he has a man and player who will prove irreplaceable - on and off the pitch.